System and method for modulating the base rate during sleep for a rate-responsive cardiac pacemaker

What is claimed is:

1. A rate-responsive pacemaker for administering pacing pulses to a patient's heart, the pacemaker including memory means for storing a predetermined transfer function that correlates sensor level measurements representative of the patient's metabolic need to heart rate, the predetermined transfer function being characterized by a base pacing rate, a maximum pacing rate, and a transition segment defining pacing rates between the base pacing rate and the maximum pacing rate, the rate-responsive pacemaker comprising:

a pulse generator for generating pacing pulses at a selectable rate between the base pacing rate and the maximum pacing rate in accordance with the predetermined transfer curve;

a physiological sensor for generating raw sensor signals indicative of the patient's level of metabolic need;

a processor, coupled to the pulse generator and the physiological sensor, for controlling the rate of pacing pulses, the processor including:

(a) means for processing the raw sensor signals to derive the sensor level measurements;

(b) means for determining an appropriate heart rate for the patient's level of metabolic need based on the sensor level measurements and the corresponding heart rate defined by the predetermined transfer function;

(c) means for causing the pulse generator to generate pacing pulses at the appropriate heart rate;

(d) means for deriving first variance measurements based on the sensor level measurements, the first variance measurements being based on the difference between a current sensor level measurement and at least one earlier sensor level measurement; and

(e) means for modulating the base pacing rate in accordance with the first variance measurements.

2. The pacemaker of claim 1, wherein:

the memory further includes means for storing a resting rate and a sleeping rate that is lower than the resting rate; and

the modulating means includes means for modulating the base pacing rate between the resting rate and the sleeping rate.

3. The pacemaker of claim 2, wherein:

the memory further includes means for storing a sensor measurement threshold and a variance measurement threshold; and

the processor includes means for setting the base pacing rate to the sleeping rate when a current sensor measurement is below the sensor measurement threshold and a current first variance measurement is below the variance measurement threshold.

4. The pacemaker of claim 3, wherein the processor includes means for setting the base pacing rate to the resting rate when either the current sensor measurement meets or exceeds the sensor measurement threshold, or the current first variance measurement meets or exceeds the variance measurement threshold.

5. The pacemaker of claim 3, wherein the processor includes means for maintaining a running average of the sensor measurements in the memory, the running average defining the sensor measurement threshold.

6. The pacemaker of claim 5, wherein the processor includes means for maintaining the running average by digitally filtering the sensor measurements using a time constant of about 18 hours.

7. The pacemaker of claim 3, wherein:

the memory includes means for storing first variance measurements derived during a predetermined period of time as a histogram, the histogram including a plurality of bins ranging from a lowest bin to a highest bin, each bin being associated with a different first variance measurement value; and

the processor includes means for deriving the variance measurement threshold by determining a bin associated with the highest first variance measurement derived while the patient was sleeping.

8. The pacemaker of claim 7, wherein:

the memory further includes means for storing a value representing a fraction of time that the patient sleeps each day;

the processor includes means for maintaining a value in the memory representing a total number of first variance measurements stored in the memory during the predetermined period of time; and

the processor includes means for deriving the highest first variance measurement derived while the patient was sleeping by:

estimating a number of first variance measurements that were derived while the patient was sleeping during the predetermined period of time in accordance with the stored fraction of time that the patient sleeps each day and the stored total number of first variance measurements; and

counting the first variance measurements stored in the histogram, starting with the lowest bin and proceeding through successively higher bins until the estimated number of first variance measurements that were derived while the patient was sleeping have been counted, the bin associated with the last count defining the highest first variance measurement derived while the patient was sleeping.

9. The pacemaker of claim 7, wherein the patient predetermined period of time is about one week.

10. The pacemaker of claim 1, wherein the processor includes means for deriving each first variance measurement by taking an absolute value of a difference between a current sensor measurement and an earlier sensor measurement, and filtering the result with a recursive, low pass digital filter.

11. The pacemaker of claim 10, wherein the current sensor measurement is derived by the processor about 26 seconds after the processor derives the earlier sensor measurement.

12. The pacemaker of claim 1, wherein:

the memory includes means for storing a base rate slope and a sleeping rate that defines a minimum heart rate maintained by the pacemaker; and

the the base pacing rate by increasing the base pacing rate from the sleeping rate by an amount defined by the product of the base rate slope and the first variance measurements.

13. The pacemaker of claim 12, wherein:

the sensor measurements derived by the processor comprise second variance measurements; and

the processor includes means for correlating the second variance measurements to the transition segment of the transfer function to derive to determine the appropriate heart rate for the patient's level of metabolic need.

14. The pacemaker of claim 13, wherein:

the memory further includes means for storing an activity slope that defines the transition segment of the transfer function; and

the processor includes means for deriving the appropriate heart rate by increasing the heart rate from the base pacing rate by an amount defined by the product of the activity slope and the second variance measurements.

15. The pacemaker of claim 13, wherein the first variance measurement exhibit less variation over time than the second variance measurements.

16. The pacemaker of claim 1, wherein:

the physiological sensor comprises an activity sensor; and

the sensor measurements derived by the processor represent levels of physical activity, the levels of physical activity being indicative of the levels of metabolic need.

17. A method of providing rate-responsive pacing therapy to a patient's heart in an implantable stimulation device, the implantable stimulation device including memory means for storing a predetermine transfer function that correlates sensor measurements representative of the patient's metabolic need to heart rate, the predetermine transfer function being characterized by a base pacing rate, a maximum pacing rate, and a transition segment defining pacing rates between the base pacing rate and the maximum pacing rate, the method comprising the steps of:

generating pacing pulses at a selectable rate between the base pacing rate and the maximum pacing rate in accordance with the predetermined transfer curve;

sensing the patient's level of metabolic need and generating raw sensor signals representative thereof;

processing the raw sensor signals to derive the sensor measurements;

determining the rate at which pacing pulses are generated based on the sensor measurements and the predetermined transfer function;

processing the sensor measurements to derive first variance measurements, the first variance measurements being based on the difference between a current sensor measurement and at least one previous sensor measurement; and

modulating the base pacing rate to a lower pacing rate in accordance with the first variance measurements.

18. The method of claim 17, wherein the step of modulating the base pacing rate comprises modulating the base pacing rate between a prescribed resting rate and a prescribed sleeping rate that is below the resting rate.

19. The method of claim 18, wherein the step of modulating the base pacing rate further comprises setting the base pacing rate to the sleeping rate when a current sensor measurement is below a sensor measurement threshold and a current first variance measurement is below a variance measurement threshold.

20. The method of claim 19, wherein the step of modulating the base pacing rate further comprises setting the base pacing rate to the resting rate when either the current sensor measurement meets or exceeds the sensor measurement threshold, or the current first variance measurement meets or exceeds the variance measurement threshold.

21. The method of claim 19 further comprising the step of maintaining a running average of the sensor measurements, the running average defining the sensor measurement threshold.

22. The method of claim 21, wherein the step of maintaining the running average comprises digitally filtering the sensor measurements using a time constant of about 18 hours.

23. The method of claim 19 further comprising the steps of:

storing first variance measurements derived during a predetermined period of time as a histogram, the histogram including a plurality of bins ranging from a lowest bin to a highest bin, each bin being associated with a different first variance measurement value; and

deriving the variance measurement threshold by determining a bin associated with the highest first variance measurement derived while the patient was sleeping.

24. The method of claim 23, wherein the step of deriving the variance measurement threshold comprises:

estimating a number of first variance measurements that were derived while the patient was sleeping during the predetermined period of time in accordance with a prescribed fraction of time that the patient sleeps each day and a total number of first variance measurements stored in the histogram during the predetermined period of time; and

counting the first variance measurements stored in the histogram, starting with the lowest bin and proceeding through successively higher bins until the estimated number of first variance measurements that were derived while the patient was sleeping have been counted, the bin associated with the last count defining the highest first variance measurement derived while the patient was sleeping.

25. The method of claim 23, wherein the predetermined period of time is about one week.

26. The method of claim 17, wherein the step of processing the sensor measurements to derive the first variance measurements comprises taking an absolute value of a difference between a current sensor measurement and an earlier sensor measurement, and filtering the result with a recursive, low pass digital filter.

27. The method of claim 26, wherein the current sensor measurement is derived about 26 seconds after the earlier sensor measurement.

28. The method of claim 17, wherein the step of modulating the base pacing rate comprises increasing the base pacing rate from a prescribed sleeping rate that defines a minimum maintained heart rate by an amount defined by the product of a prescribed base rate slope and the first variance measurements.

29. The method of claim 28, wherein:

the step of processing the sensor signals to derive sensor measurements comprises deriving second variance measurements; and

the step of applying the sensor measurements to the transfer function comprises the step of applying the second variance measurements to the transition segment of the transfer function to derive the appropriate heart rate for the patient's level of metabolic need.

30. The method of claim 29, wherein the step of applying the second variance measurements to the transition segment comprises increasing the heart rate from the base pacing rate by an amount defined by the product of a prescribed activity slope and the second variance measurements.

31. The method of claim 29, wherein the first variance measurements exhibit less variation over time than the second variance measurements.

32. The method of claim 17, wherein the step of sensing the patient's level of metabolic need comprises sensing levels of physical activity, the levels of physical activity being indicative of the levels of metabolic need.

33. A rate-responsive pacemaker for administering pacing pulses to a patient's heart, comprising:

a pulse generator for generating pacing pulses at a rate between a variable base rate and a maximum pacing rate;

an activity sensor for generating raw sensor signals indicative of the patient's activity level;

control means for determining the rate of pacing pulses generated by the pulse generator, the control means including:

(a) processing means for processing the raw sensor signals to determine activity measurements;

(b) detecting means for detecting when a patient is asleep based on the activity measurements; and

(c) modulating means for varying the base pacing rate between a resting rate and a sleeping rate when the patient is asleep.

34. The pacemaker of claim 33, wherein:

the processing means comprises means for processing the raw sensor measurements to determine activity level measurements and activity variance measurements, the activity variance measurements being based on the difference between a current activity level measurement and at least one earlier activity level measurement; and

the detecting means comprises means for detecting when a patient is asleep based on the activity variance measurements.

35. The pacemaker of claim 34, wherein the detecting means further comprises:

threshold determining means for determining an activity variance threshold based on the activity variance measurements, the patient being awake when a current activity variance measurement has a value above the activity variance threshold and asleep when the current activity variance measurement has a value below the activity variance threshold.

36. The pacemaker of claim 35, further comprising:

memory means for storing activity variance measurements derived during a predetermined period of time as a histogram, the histogram including a plurality of bins ranging from a lowest bin to a highest bin, each bin being associated with a different activity variance measurement value, the histogram being bimodal with a dominant mode corresponding to a time period when the patient was sleeping; and

wherein the threshold determining means includes means for deriving the activity variance threshold based on a distribution of the dominant mode of the histogram.

37. The pacemaker of claim 35, wherein the modulating means comprises:

means for setting the base pacing rate to the resting rate when either the current activity level measurement meets or exceeds the activity level measurement threshold, or the current activity variance measurement meets or exceeds the activity variance threshold.

38. The pacemaker of claim 35, wherein the modulating means comprises:

means for setting the base pacing rate to the sleeping rate when a current sensor measurement is below the sensor measurement threshold and a current first variance measurement is below the activity variance threshold.

39. The pacemaker of claim 34, further comprising:

memory means for storing a base rate slope; and

wherein the modulating means includes means for modulating the base pacing rate by increasing the base pacing rate from the sleeping rate by an amount defined by the product of the base rate slope and the activity variance measurements.

40. A rate-responsive pacemaker for administering pacing pulses to a patient's heart, comprising:

a pulse generator for generating pacing pulses at a rate between a base rate and a maximum pacing rate;

an activity sensor for generating raw sensor signals indicative of the patient's activity level; and

control means for determining the rate of pacing pulses generated by the pulse generator, the control means including:

processing means for processing the raw sensor signals to determine activity measurements; and

modulating means for continuously adjusting the base pacing rate based on the activity measurements.

Description Submit all comments and votes

BACKGROUND OF THE INVENTION

This invention relates to implantable cardiac pacemakers, and particularly to rate-responsive cardiac pacemakers. More particularly, this invention relates to a system and method for modulating the base rate by a transfer function for a rate-responsive pacemaker, between a resting rate that is suitable for the patient while awake but at rest, and a sleeping rate that meets the patient's low metabolic demands during sleep.

A pacemaker is an implantable medical device that delivers electrical stimulation pulses to cardiac tissue to relieve symptoms associated with bradycardia--a condition in which a patient cannot normally maintain a physiologically acceptable heart rate. Early pacemakers delivered stimulation pulses at regular intervals in order to maintain a predetermined heart rate--typically a rate deemed to be appropriate for the patient at rest. The predetermined rate was usually set at the time the pacemaker was implanted, although in more advanced pacemakers, the rate could be set remotely after implantation. Such pacemakers were known as "asynchronous" pacemakers because they did not synchronize pacing pulses with natural cardiac activity.

Early advances in pacemaker technology included the ability to sense the patient's natural cardiac rhythm (i.e., the patient's intracardiac electrogram, or "IEGM"). This led to the development of "demand pacemakers"--so named because they deliver stimulation pulses only as needed by the heart. Demand pacemakers are capable of detecting a spontaneous, hemodynamically effective cardiac contraction which occurs within a predetermined time period (commonly referred to as the "escape interval") following a preceding contraction. When a naturally occurring contraction is detected within the escape interval, the demand pacemaker does not deliver a pacing pulse. The ability of demand pacemakers to avoid delivery of unnecessary stimulation pulses is desirable because pacing pulse inhibition extends battery life and avoids competition with the patient's intrinsic rhythm.

Modern demand pacemakers allow physicians to telemetrically adjust the length of the escape interval, which has the effect of altering the heart rate maintained by the device. However, in early devices, this flexibility only allowed for adjustments to a fixed programmed rate, and did not accommodate patients who required increased or decreased heart rates to meet changing physiological requirements during periods of elevated or reduced physical activity. Therefore, unlike a person with a properly functioning heart, a patient receiving therapy from an early demand pacemaker was paced at a constant heart rate--regardless of the level to which the patient was engaged in physical activity. Thus, during periods of elevated physical activity, the patient was subject to adverse physiological consequences, including lightheadedness and episodes of fainting, because the heart rate was forced by the pacemaker to remain constant.

The adverse effects of constant rate pacing lead to the development of "rate-responsive pacemakers" which can automatically adjust the patient's heart rate in accordance with metabolic demands. An implanted rate-responsive pacemaker typically operates to maintain a predetermined minimum heart rate when the patient is engaged in physical activity at or below a threshold level, and gradually increases the maintained heart rate in accordance with increases in physical activity until a maximum rate is reached. Rate-responsive pacemakers typically include processing circuitry that correlates measured physical activity to an appropriate heart rate. In many rate-responsive pacemakers, the minimum heart rate, maximum heart rate, and a slope defining transition rates between the minimum heart rate and the maximum heart rate, are parameters that may be telemetrically adjusted to meet the needs of a particular patient.

One approach that has been considered for enabling rate-responsive pacemakers to determine an appropriate heart rate involves the use of a physiological parameter that reflects the patient's level of metabolic need. Physiological parameters that have been considered include central venous blood temperature, blood pH level, QT time interval and respiration rate. However, certain drawbacks (such as slow response time, unpredictable emotionally-induced variations, and wide variability across individuals) render the use of these physiological parameters difficult, and accordingly, they have not been widely used in practice.

Rather, most rate-responsive pacemakers employ sensors that transduce mechanical forces associated with physical activity--the level of physical activity being indicative of the patient's level of metabolic need. These activity sensors generally contain a piezoelectric transducing element which generates a measurable electrical potential when a mechanical stress resulting from physical activity is experienced by the sensor. By analyzing the signal from a piezoelectric activity sensor, a rate-responsive pacemaker can determine how frequently pacing pulses should be applied to the patient's heart.

Piezoelectric elements for activity sensors are commonly formed from piezoelectric crystals, such as quartz or barium titanite. Recently, however, activity sensors have been designed which use thin films of a piezoelectric polymer, such as polyvinylidene fluoride (commonly known by the trademark KYNAR, owned by ATOCHEM North America), rather than the more commonly used piezoelectric crystals. Activity sensors so designed are described in copending, commonly-assigned U.S. patent applications Ser. No. 08/059,698, filed May 10, 1993, now U.S. Pat. No. 5,383,473 entitled "A Rate-Responsive Implantable Stimulation Device Having a Miniature Hybrid-Mountable Accelerometer-Based and Method of Fabrication," and Ser. No. 08/091,850, filed Jul. 14 1993, now U.S. Pat. No. 5,425,750 entitled "Accelerometer-Based Multi-Axis Physical Activity Sensor for a Rate-Responsive Pacemaker and Method of Fabrication," which are hereby incorporated by reference in their entireties.

A variety of signal processing techniques have been used to process the raw sensor signals provided by activity sensors. For example, in one approach, the raw signals are rectified and filtered. Alternatively, the frequency at which the highest peaks in the signals occur can be monitored. Regardless of the particular method used, the result is typically a digital signal that is indicative of the level of sensed activity at a given time. In one preferred approach, the digital signal is produced by repeatedly integrating the raw sensor signals until a predetermined threshold value is reached. Each time the threshold is reached, a digital trigger pulse is generated. A counter is used to count the number of trigger pulses that occur in a fixed period of time (e.g., the number of trigger pulses that occur during an approximately 100 ms period within each heartbeat interval). The count reached at the end of the fixed period of time is provided to processing circuitry in the pacemaker, which processing circuitry typically includes a microprocessor.

The processing circuitry then uses the count signal to produce an activity level measurement that represents the patient's activity level. The appropriate rate at which the patient's heart is to be stimulated (known as the sensor-indicated rate) is determined by applying a transfer function to the activity level measurement. The transfer function defines a sensor-indicated rate for each possible activity level measurement.

An example of a rate-responsive pacemaker in which a transfer function is used to calculate the sensor-indicated rate is described in commonly-assigned U.S. Pat. No. 5,074,302 of Poore et al. ("the '302 patent"), which is hereby incorporated by reference in its entirety. As described therein, when relatively little activity is detected, the activity level measurement is ordinarily below a low activity threshold. When the activity level measurement is below the low activity threshold, the sensor-indicated rate is set to a base pacing rate (e.g., 60 beats per minute (bpm)), as defined by the transfer function. At high levels of measured activity, the activity level measurement may exceed a high activity threshold. When this occurs, the sensor-indicated rate is limited to a maximum pacing rate, so that the patient's heart is not stimulated too rapidly. If the value of the activity level measurement falls between the low and high activity thresholds, the pacemaker applies pacing pulses to the patient's heart in accordance with the rate determined by the transfer function, generally at a rate somewhere between the base pacing rate and the maximum pacing rate.

Typically, for activity level measurements between the low and high thresholds, the transfer function is linear. The slope of the transfer function determines increases (or decreases) in the pacing rate corresponding to a given increase (or decrease) in the activity level measurement. The larger the slope, the more rapidly the pacing rate will increase (or decrease).

The slope of the transfer function in typical rate-responsive pacemakers is telemetrically adjustable by a physician, so that the operation of a pacemaker can be tailored to suit an individual patient's needs. During follow-up visits, the slope may be adjusted by the physician if the patient's condition warrants a change. However, for some patients, more frequent slope adjustments may be desirable. In view of this need, pacemakers have been designed which can automatically adjust the slope of the transfer function. The '302 patent describes one such approach--in which high and low averages of activity sensor readings are used in connection with preprogrammed base and maximum pacing rates to derive an appropriate slope for the transfer function.

Another approach for automatically adjusting the slope of the transfer function is described in commonly-assigned, copending U.S. patent application Ser. No. 08/255,194, filed Jun. 7, 1994, entitled "System and Method for Automatically Determining the Slope of a Transfer Function for a Rate-Responsive Cardiac Pacemaker," which is hereby incorporated by reference in its entirety. The approach described therein uses the patient's activity profile, as represented by an activity level histogram stored in the pacemaker's memory, to adjust the slope of the transfer function. The activity level histogram collects activity level measurements over a predetermined period of time, preferably about a week. Each week, the activity level histogram is evaluated to determine if a slope adjustment is warranted. If an adjustment is deemed to be appropriate, the activity level histogram is used, in connection with preprogrammed base and maximum pacing rates, to define the new slope. The activity level histogram is then cleared so that new data may be collected for the next adjustment cycle. In addition, the pacemaker described in copending U.S. patent application Ser. No. 08/255,194, filed Jun. 7, 1994, advantageously inhibits slope adjustment if it is determined that the patient was bedridden for a significant portion the most recent data collection cycle (i.e., the previous week). Further, the above copending U.S. patent application, Ser. No. 08/255,194, filed Jun. 7, 1994, describes an approach that can be used to determine a slope that accommodates a patient's regular exercise routine.

The advances described in the '302 patent and the above copending application Ser. No. 08/255,194, filed Jun. 7, 1994, have lead to the development of extremely flexible pacemakers that enable bradycardia patients to achieve a level of cardiac performance that closely resembles that of healthy individuals. However, there are certain areas in which flexibility can be improved even further. For example, in most rate-responsive pacemakers, the base pacing rate (which, as described above, defines the minimum heart rate maintained by the pacemaker) is usually set telemetrically by the physician in connection with the implantation procedure and then afterward, as needed, during follow-up visits. The base pacing rate is usually set at a rate that comfortably meets the patient's metabolic needs for when the patient is awake but relatively inactive.

While a healthy individual is awake but relatively inactive, the individual's heart rate is usually maintained at a "resting rate." During sleep, the heart rate of a healthy individual typically drops to a "sleeping rate" that is below the resting rate. In this respect, pacemaker-assisted cardiac performance usually differs from what is ordinarily experienced by healthy individuals. More precisely, the fixed base pacing rate of the pacemaker (which is analogous to a healthy individual's resting rate) prevents the patient from experiencing a sleeping rate, which if available, may be more comfortable for the patient during sleep.

The difference between the sleeping rate and the resting rate for healthy individuals is usually rather small (typically in the range from about 10 bpm to about 20 bpm). However, the inability of some pacemakers to maintain a sleeping rate may cause the patient to have some difficulty falling asleep, and may occasionally lead to a restless night of sleep. In addition, since it is likely that a sleeping pacemaker patient being paced at a resting rate could withstand (and even benefit from) a lower sleeping rate, the pacemaker wastes limited energy reserves by maintaining the unnecessarily high resting rate.

In view of the foregoing, it would be desirable if the base pacing rate of a rate-responsive pacemaker could be modulated between a resting rate that is suitable for the patient while awake but relatively inactive, and a sleeping rate that meets the patient's low metabolic demands during sleep. It would also be desirable if the base pacing rate could gradually transition between a sleeping rate and a resting rate, so that abrupt rate changes as the patient transitions between sleep and wakefulness can be avoided.

SUMMARY OF THE INVENTION

In accordance with the present invention, a system and method are provided for modulating the base pacing rate of a rate-responsive pacemaker between a resting rate that is suitable for the patient while at rest but awake, and a sleeping rate that meets the patient's low metabolic demands during sleep.

In a preferred embodiment, the rate-responsive pacemaker includes a conventional pacemaker circuit that is capable of generating pacing pulses in any of the known pacing modes in accordance with instructions provided by processing circuitry (which preferably includes a microprocessor). Pacing pulses are delivered to the patient's heart through at least one conventional pacing lead, which is also used to sense natural cardiac activity when pacing pulses are not being delivered. By sensing natural cardiac activity, the pacemaker is capable of operating in a demand mode, which advantageously extends battery life.

The processing circuitry regulates the operation of the pacemaker circuit in accordance with control routines and parameters that are stored in a memory. The control routines and parameters may be modified by a physician through the use of an external programmer which communicates with the pacemaker through a telemetry circuit included within the pacemaker. In addition, the telemetry circuit may be used to communicate information from the pacemaker to the external programmer, such information including cardiac activity sensed by the pacing lead.

The rate-responsive pacemaker includes a sensor for measuring metabolic need--preferably an activity sensor that measures the patient's level of physical activity at any given time. Preferably, the activity sensor contains a piezoelectric element that generates a measurable electrical potential when a mechanical stress resulting from physical activity is experienced by the sensor. Suitable activity sensors are described in the above-incorporated U.S. patent applications Ser. Nos. 08/059,698 and 08/091,850, now U.S. Pat. Nos. 5,383,473 and 5,425,750, respectively, although other types of activity sensors may be used.

The processing circuitry determines the appropriate pacing rate by applying the activity level measurements to a transfer function. The transfer function may be generally characterized by a base pacing rate for low activity levels, a maximum pacing rate for high activity levels, and a slope that defines pacing rates between the base pacing rate and the maximum pacing rate.

When an activity level measurement is below a low activity threshold, the pacemaker circuit is instructed by the processing circuitry to generate pacing pulses at the base pacing rate as needed by the patient's heart. When an activity level measurement exceeds a high activity threshold, the pacemaker circuit is instructed to generate pacing pulses at the maximum pacing rate, as needed.

The rate at which the rate-responsive pacemaker applies pacing pulses to the patient's heart preferably varies linearly in the region between the base pacing rate and the maximum pacing rate, and is therefore characterized by a slope. (Although a linear relationship between the base pacing rate and the maximum pacing rate is preferred, other relationships may also be used without departing from the spirit of the invention.) The slope of the transfer function allows the pacemaker to deliver pacing pulses at a variable rate in accordance with variations in the activity level measurements. If the pacemaker is operating in accordance with a transfer function that has a gradual slope, changes in the pacing rate will accordingly be more gradual than if the slope were steeper.

Preferably, the maximum pacing rate is telemetrically programmed by the physician. The slope (or other relationship) defining pacing rates between the base pacing rate and the maximum pacing rate may also be programmed by the physician. Alternatively, the slope may be initially programmed by the physician and then automatically adjusted thereafter, as described in the above-incorporated U.S. patent application Ser. No. 08/255,194, filed Jun. 7, 1994.

The base pacing rate may also be telemetrically programmed by the physician and, as described above, it is typically set at a resting rate (e.g., 65 bpm) that is appropriate for when the patient is awake but relatively inactive. However, in accordance with the principles of the present invention, the base pacing rate is not necessarily fixed at the resting rate. Rather, the base pacing rate is modulated between the resting rate and a sleeping rate that is lower than the resting rate. The sleeping rate is preferably set by the physician to a rate that comfortably meets the patient's low metabolic demands during sleep (e.g., 55 bpm).

The present invention provides three approaches for modulating the base pacing rate. In the first approach, the processing circuitry uses information derived from the activity level measurements to determine when the patient falls asleep. After the patient falls asleep, the processor switches the heart rate maintained by the pacemaker from the preprogrammed resting rate to the preprogrammed sleeping rate. While the pacemaker is maintaining the patient's heart rate at the sleeping rate, the processing circuitry continues to monitor the information derived from the activity level measurements to determine when the patient awakens. When the processing circuitry determines that the patient is no longer sleeping, it switches the pacing rate back to the resting rate.

The processing circuitry performs a two part test to determine whether the patient is sleeping. First, the processing circuitry determines if the current activity level measurement is below a low activity threshold. The low activity threshold may be preprogrammed by the physician, but preferably, the low activity threshold is defined by maintaining a running average of the activity level measurements. Since the typical patient is relatively inactive most of the time, the running average approximately corresponds to the patient's resting activity level.

Second, the processing circuitry derives an activity variance measurement from the activity level measurements, and determines if the derived activity variance measurement is below an activity variance threshold. Although the activity variance threshold can also be preprogrammed by the physician, the processor preferably uses an activity variance histogram stored in the pacemaker's memory, along with a preprogrammed parameter that defines the number of hours the patient typically sleeps each day, to determine the activity variance threshold on a periodic basis. The processor uses the preprogrammed number of daily sleep hours to estimate the highest bin of the activity variance histogram that contains activity variance measurements that were derived during sleep. The selected bin defines the activity variance threshold. Preferably, the activity variance histogram contains activity variance measurements collected over a period of about a week.

If the current activity level measurement and the current activity variance measurement are below their respective thresholds, the patient is deemed to be sleeping, and accordingly, the processing circuitry sets the pacing rate to the sleeping rate. Otherwise, the processing circuitry selects a pacing rate in accordance with either the resting rate, the currently active slope of the transfer function, or the maximum pacing rate.

In the second approach for modulating the base pacing rate, the processing circuitry uses a three-term transfer relation that essentially defines the transfer function of the pacemaker (except for the maximum pacing rate, which is programmable). The transfer relation receives as input a sleeping rate, a base rate slope, an activity variance measurement, an activity slope, and an activity level measurement--and provides a heart rate as an output.

A first term of the transfer relation defines the sleeping rate which, as described above, is set to a rate that comfortably meets the patient's metabolic demands during sleep.

The second term is the product of the activity variance measurement (derived by the processor from the activity level measurements) and the base rate slope (which is preprogrammed by the physician). The purpose of the second term is to gradually increase the base pacing rate to rates above the sleeping rate defined by the first term. The base rate slope is preferably set at a rather low level--such that the second term typically contributes only about 0-15 bpm to the base pacing rate. Thus, in accordance with this approach, the base pacing rate is modulated in accordance with the sum of the first and second terms. Like the first approach, activity variance measurements are used to modulate the base pacing rate.

The third term is the product of the activity level measurement and the activity slope (which may be programmed by the physician or automatically selected by the processing circuitry). The third term increases the heart rate defined by the transfer relation from the base pacing rate (as defined by the sum of the first and second terms) to a rate appropriate for the patient's current level of activity.

The main advantage offered by the second approach is that the base pacing rate gradually transitions from the sleeping rate to rates that are appropriate when the patient is awake but relatively inactive. Also, this approach allows the base pacing rate to be set to rates between the sleeping rate and a resting rate, which may be appropriate if the patient is extremely inactive while awake (e.g., bedridden). Another advantage is that there is no need to maintain an activity variance histogram, thereby conserving limited memory space. Although the second approach does not use a fixed resting rate, for practical purposes, the gradual base rate slope effectively limits the base pacing rate to a rate appropriate for when the patient is awake but relatively inactive.

The third approach to modulating the base pacing rate is similar to the second approach, in that a three-term transfer relation defines the transfer function of the pacemaker. The first two terms of the transfer relation define the base pacing rate in substantially the same manner as that described for the second approach. However, in this approach, the third term uses activity variance measurements, instead of activity level measurements, to determine the amount by which the base pacing rate should be increased to accommodate heightened levels of activity.

The activity variance measurements used for the third term are digitally filtered using a relatively short time constant (e.g., about 1.6 minutes). In contrast, the activity variance measurements used for the second term are digitally filtered using a relatively long time constant (e.g., about 38 minutes). The longer time constant results in a filtered activity variance measurement that is resistant to short-term fluctuations in the patient's activity level. This, in turn, causes the first and second terms of the transfer relation to yield a stable base pacing rate that gradually transitions between the sleeping rate and a resting rate. The shorter time constant used to derive the activity variance measurements for the third term allows the transfer relation to respond rapidly when the patient engages in physical activity.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference numerals refer to like parts throughout, and in which:

FIG. 1 is a block diagram of a rate-responsive pacemaker which can modulate the base pacing rate of a transfer function in accordance with the principles of the present invention;

FIG. 2 generally illustrates the manner by which the rate-responsive pacemaker of FIG. 1 can modulate the base pacing rate of the transfer function in accordance with the principles of the present invention;

FIGS. 3 and 4 depict logic flow diagrams representing a first embodiment of a control program used by the processor shown in FIG. 1 to modulate the base pacing rate of the transfer function in accordance with the principles of the present invention;

FIG. 5 depicts an illustrative activity variance histogram used by the processor shown in FIG. 1 to modulate the base pacing rate of the transfer function in accordance with the principles of the present invention;

FIG. 6 is a graph depicting a plot of pacing rates, and a plot of activity variance measurements used by the processor shown in FIG. 1 to derive the pacing rates, in accordance with the first embodiment of the control program shown in FIGS. 3 and 4;

FIG. 7 depicts a logic flow diagram representing a second embodiment of a control program used by the processor shown in FIG. 1 to modulate the base pacing rate of the transfer function in accordance with the principles of the present invention;

FIG. 8 is a graph depicting a plot of pacing rates, and a plot of activity variance measurements used by the processor shown in FIG. 1 to derive the pacing rates, in accordance with the second embodiment of the control program shown in FIG. 7;

FIG. 9 depicts a logic flow diagram representing a third embodiment of a control program used by the processor shown in FIG. 1 to modulate the base pacing rate of the transfer function in accordance with the principles of the present invention; and

FIG. 10 is a graph depicting a plot of pacing rates, and plots of activity variance measurements used by the processor shown in FIG. 1 to derive the pacing rates, in accordance with the third embodiment of the control program shown in FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIG. 1, a block diagram representing a rate-responsive pacemaker 20 configured in accordance with the principles of the present invention is described. In many respects, the pacemaker 20 operates in a conventional manner to provide pacing pulses at a rate that comfortably meets the patient's metabolic demands. More precisely, the pacemaker 20 uses signals generated by a piezoelectric physical activity sensor 22 to determine the extent to which the patient is engaged in physical activity--the measured level of activity being indicative of metabolic need.

Any sensor that provides a suitable response to physical activity may be used as the sensor 22, including the activity sensors described in the above-incorporated U.S. patent applications Ser. Nos. 08/091,850 and 08/059,698, now U.S. Pat. Nos. 5,425,750 and 5,383,473, respectively. The principles of the present invention may also be applied to pacemakers which use other physiologic sensors to measure metabolic demand, such as blood oxygen sensors, pH sensors, temperature sensors, etc.

The signals generated by the sensor 22 are initially received by a sensor circuit 24. The sensor circuit 24 initially processes the raw signals generated by the sensor 22 to provide digital sensor signals to a processor 26 (which preferably includes a microprocessor (not shown)). In a preferred embodiment, the sensor circuit 24 repeatedly integrates the raw sensor signals from the sensor 22 until a predetermined threshold is reached. Each time the threshold is reached, a digital trigger pulse is generated that increments a counter (not shown) of the sensor circuit 24.

The processor 26 determines the patient's level of activity by periodically examining the contents of the counter of the sensor circuit 24. Preferably, the processor 26 examines the contents of the counter once each heartbeat interval. To conserve power, the sensor circuit 24 is preferably powered for only a small fraction of each heartbeat interval. For example, the sensor circuit 24 may be powered for approximately 100 ms during each heartbeat interval. At the end of the 100 ms period, the processor 26 reads the contents of the counter and then resets the counter for the next heartbeat interval. The processor 26 uses the values read from the counter to derive activity level measurements, in a manner described in greater detail below. Preferably, the sensor 22 and the sensor circuit 24 are calibrated such that about 416 counts corresponds to about one unit of gravity (G) measured by the sensor 22.

In addition to the sensor 22, the sensor circuit 24, and the processor 26, the pacemaker 20 includes a pacemaker circuit 28 (which may be conventional), and a memory 30 coupled to the processor 26. The pacemaker circuit 28